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Curiosity rover detects Martian methane burps

By Lisa Grossman

Mars rover selfie!

(Image: NASA/JPL-Caltech/MSSS)

Now you don’t, now you see it. NASA’s Curiosity rover has sniffed out short-lived bursts of methane on Mars, contradicting its initial methane-less reports. The find suggests that something is actively producing the gas, although it is unclear whether it is alien microbes or simply underground water interacting with rock.

“They’re very exciting measurements,” says Christopher Webster at NASA’s Jet Propulsion Laboratory in Pasadena, California, who presented the results at the meeting of the American Geophysical Union in San Francisco today. “They’ve completely opened up the debate again on Mars methane.”

The rover also found evidence of more complex organic compounds in powdered samples of rock – the first definitive detection of organics on the surface of Mars. In addition, it showed that despite recent findings that its landing site was once a habitable lake, Mars has been losing water for billions of years. Another NASA spacecraft, the MAVEN orbiter, is trying to figure out how.

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Martian methane has been playing whack-a-mole with scientists for the past decade. Detecting it would be exciting because around 90 per cent of Earth’s methane comes from living things that eat or produce methane, or the remnants of past life. In 2004, three independent teams using ground-based telescopes and orbiters around Mars reported large amounts of methane in the planet’s atmosphere, and an analysis of 2003 data from telescopes in Hawaii saw distinct plumes of methane coming from three regions on Mars.

But more recent searches by orbiting spacecraft found no sign of methane at all, meaning either it had vanished over the course of a few years – which should be impossible, as methane is thought to be stable in the atmosphere for nearly 300 years – or that it was never there to begin with.

Curiosity, which landed in a region called Gale Crater in August 2012, was supposed to make conclusive measurements. The rover carries a laser spectrometer that can determine the contents of deep breaths of Martian air.

Burp from below

In November last year, after 8 months of finding essentially no methane at all, Curiosity took a breath that contained 5.5 parts per billion of methane.

“We were completely surprised,” Webster says. Over the next two months, the methane levels jumped as high as 9 parts per billion, before falling back to about 0.7 parts per billion.

“We thought we had written the last chapter in the book of methane on Mars last year,” Webster says. “And then suddenly this happened a year ago, and we’ve seen it jump up and go away again.”

“I’m going to go out on a limb here and say flat-out, I consider this to be the first truly credible detection of methane in the atmosphere,” says Bruce Jakosky of the University of Colorado, Boulder, who is the principal investigator on the MAVEN mission. “A lot of questions have been raised about Earth-based telescopic measurements. This is incontrovertible. I don’t see any way it could not be correct. That makes it a very profound measurement.”

But it is also a perplexing one. The fact that the methane spiked and then disappeared again means it probably came from a small source somewhere near the rover – a larger plume farther away would have diffused by the time it reached the rover and made the background levels higher, Webster reasons.

Then there’s the matter of what is producing it. It could just as easily be from methane-producing microbes living beneath Gale Crater as from an underground aquifer interacting with buried rocks.

“We’re still stuck with the fact that methane has multiple possible sources,” Jakosky says. “We don’t have information yet to tell.” MAVEN doesn’t have enough sensitivity to methane, but the Russian/European ExoMars Trace Gas Orbiter, due to launch in 2016, might be able to help.

The team also reported that sample drilled from a mudstone dubbed Cumberland contained an organic compound called chlorobenzene, which is used on Earth in the production of the pesticide DDT. The compound might not be present in the rock in that form – it could have formed when it was heated inside Curiosity’s on-board chemistry laboratory, possibly by interaction with perchlorate compounds in Martian soil.

It’s also not necessarily a sign of past life on Mars, notes Curiosity project scientist John Grotzinger of the California Institute of Technology. But it does indicate that it’s possible to preserve fragile organic compounds in Martian rocks, a good sign for future missions like NASA’s planned rover in 2020.

Heavy hydrogen

In another presentation at AGU, Paul Mahaffy of NASA Goddard Spaceflight Center in Maryland reported new measurements of Gale Crater’s deuterium content, an isotope of hydrogen that indicates how much water the planet once had. Deuterium atoms contain an extra neutron, making them heavier than regular hydrogen atoms. On Earth, deuterium is relatively rare, and scientists expect that Mars started out with a similar deuterium-to-hydrogen ratio. But by heating a soil sample to between 550 ⁰C and 950 ⁰C and analysing the vapour it released, Curiosity has found that the deuterium-to-hydrogen ratio in 3 billion year-old soils is three times that of Earth’s.

That means much of the lighter form of hydrogen has been lost over time, Mahaffy says. And because we have deuterium ratios from meteorites that predate the soils, we know that Mars has been losing water throughout its history, not in one big cataclysmic event.

“We’re filling a point in Mars history that has not been sampled by meteorite studies,” Mahaffy says. “We’re getting a value that is halfway between what the current atmosphere is and what it was way back when, which plausibly could have been very close to what the Earth’s oceans are.”

But it’s hard to know how much water there was globally back then, because we don’t know how quickly Mars is losing hydrogen today, says Jakosky. “That’s where MAVEN will come in, trying to get that side of the coin.”